The present invention relates to an exhaust emission control system for an internal combustion engine, and more particularly to an exhaust emission control system including a NOx removing device for removing NOx (nitrogen oxides) and having a function of determining deterioration of the NOx removing device.
When the air-fuel ratio of an air-fuel mixture supplied to an internal combustion engine is set in a lean region with respect to a stoichiometric ratio (i.e., in the case of carrying out a so-called lean operation), the emission amount of NOx tends to increase. To cope with this, a known technique for exhaust emission control includes providing a NOx removing device containing a NOx absorbent for absorbing NOx in the exhaust system of the engine. The NOx absorbent has a characteristic that when the air-fuel ratio is set in a lean region with respect to the stoichiometric ratio and the oxygen concentration in exhaust gases is therefore relatively high (the amount of NOx is large) (this condition will be hereinafter referred to as xe2x80x9cexhaust lean conditionxe2x80x9d), the NOx absorbent absorbs NOx. When the air-fuel ratio is set in a rich region with respect to the stoichiometric ratio and the oxygen concentration in exhaust gases is therefore relatively low (this condition will be hereinafter referred to as xe2x80x9cexhaust rich conditionxe2x80x9d), the NOx absorbent discharges the absorbed NOx. The NOx removing device containing this NOx absorbent is configured so that NOx discharged from the NOx absorbent in the exhaust rich condition is reduced by HC and CO and then exhausted as nitrogen gas, while HC and CO are oxidized by NOx and then exhausted as water vapor and carbon dioxide.
There is naturally a limit to the amount of NOx that can be absorbed by the NOx absorbent, and this limit tends to decrease with deterioration of the NOx absorbent. A technique of determining a degree of deterioration of the NOx absorbent is known in the art (Japanese Patent Laid-open No. Hei 10-299460). In this technique, two oxygen concentration sensors are arranged upstream and downstream of the NOx removing device, and air-fuel ratio enrichment for discharging the NOx absorbed by the NOx absorbent is carried out. Then, the degree of deterioration of the NOx absorbent is determined according to a delay time period from the time when an output value from the upstream oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio to the time when an output value from the downstream oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio.
However, the above delay time period changes with engine operating condition, (engine rotational speed and engine load), so that unless the engine operating condition for the execution of deterioration determination is limited to a narrow range, an improper determination is likely. Changing the deterioration determination threshold according to the engine operating condition is one method for solving this problem. However, improper determination is prone to occur in transient conditions where the engine operating condition changes.
Further, the NOx absorbing capacity of the NOx absorbent is reduced not only by the deterioration of the NOx absorbent, but also by absorption of sulfur oxides (SOx) contained in fuel components. This absorption is called sulfur poisoning. In the conventional deterioration determining technique, however, sulfur poisoning is not taken into consideration, Therefore, there is a possibility that it may be improperly determined that the NOx absorbent itself is deteriorated when the NOx absorbing capacity is reduced by sulfur poisoning. The NOx absorbing capacity reduced by sulfur poisoning can be recovered by regeneration processing. Accordingly, it is undesirable to determine that the NOx absorbent is in an unrecoverable deteriorated condition in the case of sulfur poisoning.
It is accordingly an object of the present invention to provide an exhaust emission control system which can perform accurate determination of deterioration of a NOx removing device regardless of an engine operating condition.
It is another object of the present invention to provide an exhaust emission control system which can accurately determine deterioration of a NOx removing device and has a function of reliably regenerating the NOx removing device in the case that it is regenerable.
In accordance with an aspect of the present invention, there is provided an exhaust emission control system for an exhaust system for an internal combustion engine, having a nitrogen oxide removing means for absorbing NOx contained in exhaust gases in an exhaust lean condition. This system further comprises the following: first and second oxygen concentration sensors respectively provided upstream and downstream of the nitrogen oxide removing means for detecting the oxygen concentration in the exhaust gases; deterioration-determination enriching means for changing the air-fuel ratio of an air-fuel mixture to be supplied to the engine from a lean region to a rich region with respect to a stoichiometric ratio; reducing-component amount calculating means for calculating an amount of reducing components flowing into the nitrogen oxide removing means from the time when an output value from the first oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio after the enrichment executed by the deterioration determination enriching means; and deterioration determining means for determining deterioration of the nitrogen oxide removing means according to the amount of reducing components calculated by the reducing-component amount calculating means and an output value from the second oxygen concentration sensor.
With this arrangement, after changing the air-fuel ratio of an air-fuel mixture to be supplied to the engine from a lean region to a rich region with respect to the stoichiometric ratio, an amount of reducing components flowing into the nitrogen oxide removing means is calculated from the time when the output value from the first oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio, and the deterioration of the nitrogen oxide removing means is determined according to the calculated amount of reducing components and the output value from the second oxygen concentration sensor. Accordingly, the deterioration determination can be performed according to the amount of reducing components changing with an engine operating condition to thereby make it possible to perform accurate determination of deterioration of the nitrogen oxide removing means in a wide range of engine operating conditions.
Preferably, the deterioration determining means determines that the nitrogen oxide removing means is deteriorated when the output value from the second oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio before the amount of reducing components reaches a predetermined amount.
As a modification, the deterioration determining means determines that the nitrogen oxide removing means is deteriorated when the amount of reducing components at the time the output value from the second oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio is smaller than a predetermined amount.
As another modification, the deterioration determining means determines that the nitrogen oxide removing means is deteriorated when the output value from the second oxygen concentration sensor indicates a rich air-fuel ratio at the time the amount of reducing components has reached a predetermined amount.
Preferably, the reducing-component amount calculating means calculates the amount of reducing components by integrating an amount of exhaust gases flowing into the nitrogen oxide removing means.
With this configuration, the amount of reducing components is calculated by integrating the amount of exhaust gases flowing into the nitrogen oxide removing means, so that the calculation of the reducing component amount can be easily made by using a parameter indicative of the amount of exhaust gases.
Preferably, the exhaust emission control system further includes reduction enriching means for intermittently enriching the air-fuel ratio to reduce NOx absorbed by the nitrogen oxide removing means. In this system, the deterioration-determination enriching means executes enrichment of the air-fuel ratio with a degree of enrichment smaller than a degree of enrichment executed by the reduction enriching means over a time period longer than a time period of enrichment executed by the reduction enriching means.
Preferably, the reducing-component amount calculating means uses a basic fuel amount as a parameter indicative of the amount of exhaust gases flowing into the nitrogen oxide removing means, the basic fuel amount being set according to a rotational speed of the engine and an intake air pressure of the engine so that the air-fuel ratio becomes a substantially constant value.
Preferably, the first and second oxygen concentration sensors have characteristics that the output values therefrom rapidly change in the vicinity of the stoichiometric ratio.
Preferably, the deterioration determining means makes the determination whether the nitrogen oxide removing means is normal or deteriorated, or makes the decision of withholding the determination, according to the amount of reducing components calculated by the reducing-component amount calculating means and the output value from the second oxygen concentration sensor, and the control system further includes alarming means for giving an alarm when the determination that the nitrogen oxide removing means is deteriorated has been made by the deterioration determining means; and deterioration regenerating means for executing regeneration processing for the nitrogen oxide removing means when the decision of withholding the determination has been made by the deterioration determining means.
With this arrangement, the determination whether the nitrogen oxide removing means is normal or deteriorated, or the decision of withholding this determination is made according to the amount of reducing components calculated by the reducing-component amount calculating means and the output value from the second oxygen concentration sensor. When it is determined that the nitrogen oxide removing means is deteriorated, an alarm is given to a driver, whereas when the decision of withholding the determination is made, the regeneration processing for the nitrogen oxide removing means is executed. Accordingly, the driver can immediately take certain measures when the nitrogen oxide removing means is determined to be apparently deteriorated, and as in the case that the absorbing capacity of the NOx absorbent is reduced by sulfur poisoning, the withholding of the determination is decided to allow reliable regeneration of the NOx removing device.
More preferably, the deterioration determining means determines that the nitrogen oxide removing means is deteriorated when the amount of reducing components is smaller than an NG determination threshold, determines that the nitrogen oxide removing means is normal when the amount of reducing components is greater than or equal to an OK determination threshold larger than the NG determination threshold, or decides to withhold the determination when the amount of reducing components is in the range between the NG determination threshold and the OK determination threshold, at the time the output value from the downstream oxygen concentration sensor has changed to a value indicative of a rich air-fuel ratio.
As a modification, the deterioration determining means determines that the nitrogen oxide removing means is deteriorated when the output value from the downstream oxygen concentration sensor indicates a rich air-fuel ratio at the time the amount of reducing components has become greater than or equal to an NG determination threshold, determines that the nitrogen oxide removing means is normal when the output value from the downstream oxygen concentration sensor indicates a lean airfuel ratio at the time the amount of reducing components has become greater than or equal to an OK determination threshold larger than the NG determination threshold, or decides to withhold the determination when the output value from the downstream oxygen concentration sensor indicates a rich air-fuel ratio at the time the amount of reducing components has become greater than or equal to the OK determination threshold.
Preferably, the deterioration regenerating means performs the regeneration processing for the nitrogen oxide removing means by setting the air-fuel ratio to a rich region with respect to the stoichiometric ratio in an engine operating condition where the temperature of the nitrogen oxide removing means becomes higher than a predetermined temperature.
More preferably, the deterioration regenerating means sets an execution time for the regeneration processing according to a rotational speed of the engine and an intake air pressure of the engine.